This invention relates to an imaging charged particle beam exposure system employed for writing fine patterns on wafers or mask substrates in the process of manufacturing integrated circuit devices, ion implantation, vacuum vapor deposition, etc., and more particularly to a multiple-imaging charged particle-beam exposure system, which is capable of writing identical patterns on a lot of chips at the same time.
Imaging charged particle beam pattern writing methods using a single scanning-electron beam have conventionally been employed to directly write patterns on wafers for integrated circuit devices. However, the methods suffer from low productivity. To overcome this disadvantage, a multiple-electron-beam exposure system has been proposed, which uses a plurality of charged particle beams to write identical patterns on a plurality of chips at the same time to thereby improve the productivity, for example, by IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, No. 11, Nov. 1981, pp 1422-1428.
FIG. 1 shows the construction of this conventional multiple-imaging charged particle-beam exposure system. A charged particle beam 108 is emitted from a beam source 101 via an Einzel lens 102, a blanker 103, and an object aperture 104, and deflected by a deflector 105 to uniformly flood a screen lens 106 so that a charged particle beam emerging from each of lens apertures of the screen lens 106 forms an image of the object aperture on an object (e.g. wafer) 107 under each lens aperture. In FIG. 1, Zo is a distance (hereinafter referred to as "the first distance") between the object aperture 104 and the screen lens 106, Zi a distance (hereinafter referred to as "the second distance") between the screen lens 106 and the object 107 to be exposed, V.sub.1 a voltage (hereinafter referred to as "the first voltage") between the beam source 101 and the screen lens 106, and V.sub.2 a voltage (hereinafter referred to as "the second voltage") between the beam source and the object 107.
The first distance Zo and the second distance Zi are set at approximately 1,000 mm and 20 mm, respectively. In such an arrangement, in order to focus charged particle beams on the object 107, the second voltage V.sub.2 should be approximately nine times as high as the first voltage V.sub.1 (V.sub.2 =9 V.sub.1). In the meanwhile, the first voltage V.sub.1 has to be set within a small range limited by the capacity of the beam source and the required current density (energy of the beam), and is set to 1 KV, for example. The second voltage V.sub.2 is, therefore, necessarily set to approximately 9 KV. Therefore, the conventional system is not suitable for an application other than the pattern writing, for example, ion implantation which requires high imaging charged particle beam energy and vacuum deposition which uses imaging charged particle beam having relatively low energy, if a single such system is used, because in the case of ion implantation the second voltage V.sub.2 has to be set to 100 KV whereas in the case of vacuum deposition it has to be set to 1 KV. In order to avoid this inconvenience, a method has conventionally been used, which employs an electrode for acceleration and deceleration of charged particle beams, which is arranged immediately before the object. However, the second distance Zi cannot be set to a large distance due to restrictions imposed by the required magnification (Zi/2Zo) of an image to be formed on the object 107 and the size of the system (Zi is usually approximately 20 mm). Therefore, it is difficult to simply arrange the electrode for acceleration and deceleration of beam between the screen lens 106 and the object 107.
Further, the conventional exposure system also suffers from the following problem: As shown in FIG. 2, when beams emerging from lens apertures 106a of the screen lens 106 are focused on the wafer 107, beams from lens apertures 106b, 106c in the peripheral portion of the screen lens 106 are focused before they reach the wafer 107 to form obscure images on the peripheral portion of the wafer 107. Therefore, the number of lens apertures in the screen lens 106, i.e. the number of chips to be exposed at the same time must be reduced to a range within which all the chips have clear images formed thereon.
In order to solve this problem of obscurity of images, the following exposure systems have conventionally been proposed:
(1) An exposure system, in which, as shown in FIG. 3, the screen lens 106 is curved such that the beams emerging from lens apertures 106b, 106c in the peripheral portion of the screen lens 106 are focused on the wafer 107 (Japanese Provisional Patent Publication (Kokai) No. 62-76619).
(2) An exposure system, in which a deflector is provided between the screen lens 106 and the wafer 107 to deflect beams emerging from the screen lens 106 (Japanese Provisional Patent Publication (Kokai) No. 60-173834).
In the exposure system (1), the screen lens 106 must be precisely curved in accordance with the distance between the screen lens 106 and the object aperture 104, the diameter of lens apertures, etc., which requires strict tolerances of the curvature of the screen leans. Further, since focal locations where images are formed by individual beams from the lens apertures are not each corrected, it is impossible to correct deviations of relative position between chips due to global alignment, thermal strain and machining strain produced in devices in the course of manufacture thereof, etc.
In the exposure system (2), the deflector provided between the screen lens and the wafer can disturb the magnetic field therebetween, so that the beams do not form clear images on the wafer, which results in degraded resolution.
Further, in the exposure system of FIG. 1, the Einzel lens 102 converges the beam 108 so as to pass through the object apeture 104. However, as shown in FIG. 4, part of the beam is irradiated on the perimeter (hatched part in FIG. 4) of the object aperture 104 of the object aperture plate 104'. This can cause sputtering damage to the object aperture plate 104' (formed, e.g., of molybdenum and tungsten) and hence prevents long-time continuous operation of the system.
Further, part of the beam is intercepted by the object aperture plate 104' to such a degree as to lose 40% of the energy of the beam in passing through the object aperture, so that the beam current density on the object is decreased, limiting the exposure region thereof that can be effectively exposed.